1. Field of the Invention
The present invention relates generally to formation fracturing systems for wells wherein treatment fluid, typically a slurry of a carrier liquid and sand or proppant, is pumped into an isolated perforated casing zone at sufficient pressure to fracture the surrounding production formation. Typically, proppant is caused to flow into the fractures and serves to prevent formation fractures from closing and thus maintain efficient fluid paths for propagation of production fluid from the formation to the well casing for production by tubing that is located within the casing. More particularly, the present invention concerns an isolation system that isolates the fluid column of a well service tubing string from either an overpressured or underbalanced reservoir condition.
2. Description of the Prior Art
When a fracturing treatment is performed on a zone isolated by packers possible common problems include (1) an overpressured reservoir condition and (2) an underbalanced reservoir condition. An overpressured reservoir condition is one in which the pressure of the reservoir is higher than the bottom hole pressure created by the hydrostatic column of fluid in the coiled tubing. This requires surface pressure to be applied to the coiled tubing to prevent the reservoir from producing up the coiled tubing. This applied surface pressure significantly decreases the life of the coiled tubing as it is cycled in and out of the wellbore. An underbalanced reservoir condition is one in which the hydrostatic pressure of a full column of fluid or slurry inside the coiled tubing creates a bottom hole pressure that is greater than the reservoir pressure and the casing/coiled tubing annulus pressure. The hydrostatic pressure typically results in the existence of a differential pressure across the straddle packer elements of a well service tool that prevents the tool from being moved to the next zone after having completed a well service activity at a selected zone within the casing.